On the Holds tab of the invoice workbench, how do you make the Release Reason read only?

We have a DFF on ap_holds that needs to be entered when the invoice hold is released for certain types of holds....not all of them.

Our users are entering the information for the DFF in the Release Reason cell and leaving the DFF blank. When they try to go back and correct it, they receive an error message:

The reason is because the form populates the Release Reason with the correct value but the ap_holds_all table has the value entered by the user so there is a miss-match that triggers the error message.

Is there any way to make the Release Reason a read-only cell in the form without doing a customization?